For some unknown reason. It wasn't because they got even mediocre ratings. The ratings were so low that they didn't even make the weekly reports of ratings.
FX is famous for renewing everything no matter how bad they do in the ratings.

1.2 premiere to .3 -- very troubling
best part of yesterday's Louie was seeing how much he's aged in 20 years, and Sarah looks like she's been in a spaceship near the speed of light comparatively.
besides that it probably was the weakest of all his episodes.
With Sarah and Mark guesting, you thought it should have been a lot funnier.